1. Field of the Invention
The present invention relates to seismic data acquisition, and more particularly to a method and system for transmitting data and control signals between multiple remote stations in an array. Most particularly, the invention relates to a seismic data collection system utilizing multiple, wireless, self-contained, seismic recording units or pods each having an associated wireless communications unit in close, but detached proximity thereto, the wireless communications unit having both short range and mid-range transmission capabilities.
2. Description of the Prior Art
Seismic exploration generally utilizes a seismic energy source to generate an acoustic signal that propagates into the earth and is partially reflected by subsurface seismic reflectors (i.e., interfaces between subsurface lithologic or fluid layers characterized by different elastic properties). The reflected signals are detected and recorded by seismic units having receivers or geophones located at or near the surface of the earth, thereby generating a seismic survey of the subsurface. The recorded signals, or seismic energy data, can then be processed to yield information relating to the lithologic subsurface formations, identifying such features, as, for example, lithologic subsurface formation boundaries.
Typically, the seismic units or stations are laid out in an array, wherein the array consists of a line of stations each having at least one geophone attached thereto in order to record data from the seismic cross-section below the array. For data over a larger area and for three-dimensional representations of a formation, multiple lines of stations may be set out side-by-side, such that a grid of receivers is formed. Often, the stations and their geophones are remotely located or spread apart. In land seismic surveys for example, hundreds to thousands of geophones may be deployed in a spatially diverse manner, such as a typical grid configuration where each line of stations extends for 5000 meters with stations spaced every 25 meters and the successive station lines are spaced 200 meters apart.
Various seismic data transmission systems are used to connect remote seismic acquisition units to a control station. Generally, the seismic stations are controlled from a central location that transmits control signals to the stations and collects seismic and other data, such as quality control data, back from the stations. Alternatively, the seismic stations may transmit data back to an intermediate data collection station such as a concentrator, where the data is recorded and stored until retrieved. Whichever the case, the various stations are most commonly hard wired to one another utilizing data telemetry cable. In the case of systems deployed in a marine environment, such cabling may be clad to withstand high pressure and corrosion. Commonly, cable telemetry is used for data transmission between the individual receivers, the stations and the central location. Other systems use on-board, wireless data transmission systems for communications and data transmission. Still other systems temporarily store the data at each station until the data is extracted.
In the case of wired stations, typically several geophones are connected in a parallel-series combination on a single twisted pair of wires to form a single receiver group or channel for a station. During the data collection process, the output from each channel is digitized and recorded by the station for subsequent analysis. In turn, stations are usually connected to cables used to communicate with and transport the collected data to recorders located at either a control station or a concentrator station.
In the case of wireless seismic units, each unit utilizes mid-range or long range radio transmission to communicate with either a central control station or concentrator via a transmitter on-board the seismic unit. Transmissions are made either directly between a seismic unit and the control station or directly between a seismic unit and the concentrator. To the extent the transmissions are high power, long-range signals, such as between a seismic acquisition unit and a central control station, the transmissions generally require a license from the local governing authority. Units capable of such transmissions also have higher power requirements and thus require larger battery packages. To the extent the seismic acquisition units transmit to a concentrator station utilizing a low power, mid-range signal, the transmitting and receiving units must typically have a line of site therebetween.
Those skilled in the art will understand that in order to enhance detection of seismic energy within the earth, it is necessary to maximize ground coupling between the earth and the seismic systems, and particularly the geophones of the system. Thus, it is desirable to position seismic units directly in contact with the ground and preferably, maximize the surface area of contact between the seismic unit and the ground. Moreover, it is also desirable to minimize noise that can arise from various external sources, such as wind, by minimizing the profile of the seismic unit, and specifically the height of the seismic unit as it deployed on the ground. In this regard, even an antenna projecting from the seismic unit will be subject to cross-winds and the like, thereby resulting in noise in the collected seismic energy.
One drawback to low-profile seismic units placed on the ground is that their capability of wirelessly communicating with external systems is greatly reduced, particularly if it is a line-of-sight system such as described above. This is particularly true if the seismic unit is fully or partially buried in the ground. In addition to the presence of a physical structure in the line of site between the unit and a receiver, other factors that can inhibit transmissions are a weak signal, weather conditions, topography, interference from other electrical devices operating in the vicinity of the unit, or disturbance of the unit's deployment position.
Thus, it would be desirable to provide a communication system for a seismic survey array that has flexibility in wirelessly transmitting signals and data to and from remote seismic units and a control and/or data collection station. The system should be capable of communication between functional seismic units even if one or more intermediate stations fail to operate properly. In addition, the system should be capable of communication between functional seismic units even if a change in environmental or physical conditions inhibits or prevents a direct transmission between a remote unit and its control station. The system should maximize wireless transmission capability while minimizing the possibility of noise from external sources. Similarly, the system should maximize coupling between the system and the earth.